sequencer (rebase -i): learn about the 'verbose' mode

When calling `git rebase -i -v`, the user wants to see some statistics
after the commits were rebased. Let's show some.

The strbuf we use to perform that task will be used for other things
in subsequent commits, hence it is declared and initialized in a wider
scope than strictly needed here.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

sequencer.c

@@ -63,6 +63,8 @@ static GIT_PATH_FUNC(rebase_path_stopped_sha, "rebase-merge/stopped-sha")
  * command-line (and are only consumed, not modified, by the sequencer).
  */
 static GIT_PATH_FUNC(rebase_path_gpg_sign_opt, "rebase-merge/gpg_sign_opt")
+static GIT_PATH_FUNC(rebase_path_orig_head, "rebase-merge/orig-head")
+static GIT_PATH_FUNC(rebase_path_verbose, "rebase-merge/verbose")
 
 static inline int is_rebase_i(const struct replay_opts *opts)
 {
@@ -1121,6 +1123,9 @@ static int read_populate_opts(struct replay_opts *opts)
 		}
 		strbuf_release(&buf);
 
+		if (file_exists(rebase_path_verbose()))
+			opts->verbose = 1;
+
 		return 0;
 	}
 
@@ -1493,9 +1498,26 @@ static int pick_commits(struct todo_list *todo_list, struct replay_opts *opts)
 	}
 
 	if (is_rebase_i(opts)) {
+		struct strbuf buf = STRBUF_INIT;
+
 		/* Stopped in the middle, as planned? */
 		if (todo_list->current < todo_list->nr)
 			return 0;
+
+		if (opts->verbose) {
+			const char *argv[] = {
+				"diff-tree", "--stat", NULL, NULL
+			};
+
+			if (!read_oneliner(&buf, rebase_path_orig_head(), 0))
+				return error(_("could not read '%s'"),
+					rebase_path_orig_head());
+			strbuf_addstr(&buf, "..HEAD");
+			argv[2] = buf.buf;
+			run_command_v_opt(argv, RUN_GIT_CMD);
+			strbuf_reset(&buf);
+		}
+		strbuf_release(&buf);
 	}
 
 	/*

sequencer.h

@@ -24,6 +24,7 @@ struct replay_opts {
 	int allow_empty;
 	int allow_empty_message;
 	int keep_redundant_commits;
+	int verbose;
 
 	int mainline;